The test is not scored based on the percentage correct. It may be a good idea for you to learn more about how this test works, because if you are approaching it as though the goal is to get everything right, then you're not going to maximize your score. You can learn more about the scoring by reading the Scoring section of our free e-book The GMAT Uncovered. There's already a copy sitting in your free account with us.
You mentioned that your test is in 3 days (tomorrow, now!). There's really nothing you can do in 3 days to change your score. You should expect to score approximately what you have been scoring in practice (assuming the practice tests were taken under official conditions, including the essay and IR sections, timing, length of breaks, etc.).
If you are looking for a higher score, then you have two choices. You can take the test anyway, just for the experience, but know that you'll probably have to take its again in future. Or you can cancel. If you cancel within 7 days of the test, you will lose the entire $250 fee. (For others reading this: if you cancel more than 7 days in advance, you have to pay only a $50 fee to reschedule.)
If you feel that you can mentally accept a 620-ish score without getting demoralized, then I'd go ahead and take the test tomorrow - you've already paid, so you might as well get the experience.
If you feel, though, that you may become demoralized, then you may want to skip the test entirely.
Okay, now let's talk about how to improve! First, what's your goal score?
Second, read this:
https://www.manhattangmat.com/blog/inde ... lly-tests/That's the mindset you need on the test - you're going to need to start building that.
Third, read this:
http://www.manhattangmat.com/blog/index ... -the-gmat/This is HOW you study for the test.
Finally, Use the below to analyze your most recent MGMAT CAT (this should take you a minimum of 1 hour):
http://www.manhattangmat.com/blog/index ... ice-tests/Figure out your strengths and weaknesses as well as what you think you should do based on that analysis. Then come back here and tell us; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)